It’s because physician compensation is only around 8% of medical costs and physician services billed are only 14%. So that’s your limit to how much reducing malpractice insurance premiums can reduce healthcare costs.
Also over the last 20 years physicians groups have mostly been bought up by private equity, so any actual savings just gets vacuumed up as their profit.
It hasn't had much impact on costs:
https://healtheconomicsreview.biomedcentral.com/articles/10....